1. Field of the Invention
This invention relates broadly to methods and apparatus for investigating subsurface earth formations. More particularly, this invention relates to sonic borehole tools and methods for measuring a nonlinear parameter of an earth formation. The invention has particular application in using the nonlinear parameter measurement for distinguishing between well consolidated and poorly consolidated rocks, which information is useful in the production of oil from the formation.
2. State of the Art
The art of sonic well logging for use in determining formation parameters is a well established art. Sonic well logs are typically derived from sonic tools suspended in a mud-filled borehole by a cable. The tools typically include a sonic source (transmitter) and a plurality of receivers which are spaced apart by several inches or feet. Typically, a sonic signal is transmitted from the transmitter at one longitudinal end of the tool and received by the receivers at the other, and measurements are made every few inches as the tool is drawn up the borehole. The sonic signal from the transmitter or source enters the formation adjacent the borehole, and the arrival times of the compressional (P-wave), shear (S-wave) and Stoneley (tube) waves are detected by the receivers. The receiver responses are typically processed in order to provide a time to depth conversion capability for seismic studies as well as for providing the determinations of formations parameters such as porosity.
It is well established in the sonic well logging arts that the acoustic signal which is transmitted in the borehole and the formation is composed of many different components. In particular, it is known that if the source has an axially symmetric component, at low frequencies (i.e., where the wavelength is large compared to the borehole radius), the dominant contribution to the acoustic signal is due to the lowest lying "mode" which is known as the "tube" or Stoneley wave. Typically, the tube wave is assumed to be a nondispersive wave having a speed V.sub.T which is expressed in terms of the properties of the fluid (borehole mud) and the solid (formation). Measurements of the tube wave are typically used for making permeability determinations. In addition, because tube waves are reflected by fractures in the formation, tube waves are also used in locating formation fractures for purposes of oil production.
While the measurement of the compressional, shear, and tube waves are useful in quantifying various parameters of the formation, it will be appreciated that to date, there has been no successful mechanism for making an in situ determination of the relative state of the consolidation of the formation. Whether a layer of a formation is well or poorly consolidated, however, can broadly affect the producibility of the formation, as well as the manner in which production is to be carried out.